Hearing loss is a common consequence of aging, with 70% of older Americans reporting some degree of hearing or balance disorder. Many factors contribute to this age-related hearing loss, including exposure to environmental noise, certain diseases, drug-induced ototoxicity, as well as inheritance of specific genes. Humans and animals show a wide variation in their susceptibility to noise-induced hearing loss, suggesting that additional genetic or environmental factors may contribute to this degenerative process. The experiments in this application are designed to investigate how noise exposure contributes to age-related hearing loss using an avian model. Broiler birds raised commercially for meat production show a substantial progressive age-related cochlear degeneration beginning at 30 weeks of age and producing severe cochlear damage by 66 weeks of age. Egg layer birds, raised commercially for egg production, show little or no cochlear damage at these same ages. Both breeds of birds live in a noisy environment, suggesting that broiler birds are substantially more susceptible to noise-induced hearing loss. The cochlear degeneration seen in broiler birds is even more striking because all birds have the unusual ability to regenerate new cochlear sensory cells (hair cells) following cochlear damage. We will examine the relative contributions of noise and genetics to this age-related cochlear degeneration by raising birds of both breeds in noisy and quiet environments. At specific ages we will measure distortion product otoacoustic emissions (DPOAEs) to test cochlear hair cell function, and we will use scanning electron microscopy to evaluate cochlear hair cell anatomy. By comparing these measures quantitatively between groups we will evaluate the relative contributions of noise exposure and genetics to age- related hearing loss. In a second set of experiments, we will expose groups of birds acutely to very loud noise. Using the same anatomical and physiological measures we will determine how cochlear integrity, age, and breed affect the cochlear response to short-term noise exposure. In addition to these measures of peripheral auditory function, we will examine two anatomical indices of the auditory central nervous system to determine how age-related cochlear degeneration and noise affect brain function.